Electrostatic powder cloud xerographic development method and apparatus



June 21, 1966 P. F. KING 3,257,223

ELECTROSTATIC POWDER CLOUD XEROGRAPHIC DEVELOPMENT METHOD AND APPARATUSFiled Nov. 1, 1962 2 Sheets-Sheet 1 INVENTOR. PAUL F'. KIN G ATTORNEYJune 21, 1966 P. F. KlNG 3,257,223

ELECTROSTATIC POWDER CLOUD XEROGRAPHIC DEVELOPMENT METHOD AND APPARATUSFiled Nov. 1, 1962 2 Sheets-Sheet 2 L l2 27 /9 1-1; hi/1771);; "2);,

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' INVENTOR. PAUL F. KING United States Patent 3,257,223 ELECTROSTATICPOWDER CLOUD XEROGRAPH- IC DEVELOPMENT METHOD AND APPARATUS Paul F.King, Webster, N.Y., assignor to Xerox Corporation, Rochester, N.Y acorporation of New York Filed Nov. 1, 1962, Ser. No. 234,611 8 Claims.(Cl. 117-17.5)

This invention relates in general to xerography and in particular to animproved method for the development of latent electrostatic images inxerography.

In the art of xerography as originally disclosed by Carlson in U.S.Patent 2,297,691 and as further amplified by many related patents in thefield, an electrostatic latent image is formed on a photoconductiveinsulating layer with a relatively conductive backing and is thendeveloped or made visible by the selective deposition thereon offinelydivided, electroscopic, marking material which is generallyreferred to in the art as toner. This toner image is then either fixedon the surface of the photoconductive insulating layer or transferred toan additional copy sheet where it is then fixed depending upon Whetheror not the photoconductive insulating layer is reuseable. Thephotoconductive insulating layer with its relatively conductive backing,which is referred to in the art as a plate, regardless of its shape, isin most instances first charged to sensitize it and then exposed to avisible light image or other pattern of activating electromagneticradiation which serves to dissipate the charge in radiation struckareas, leaving a charge pattern on the plate which conforms to theelectromagnetic radiation pattern applied to it. This exposure step isfollowed by the development and transfer steps described above.

As might be imagined from this brief description of the operation of thexerographic process the way in which the development of the invisiblelatent electrostatic image is carried out has a very significant effectupon the quality of the final copy produced by the process. Thus, evenif a latent electrostatic image of excellent quality is formed on thexerographic plate, good quality copy will not be produced unless aneffective developing technique is utilized. In the process of attemptingto find good xerographic developing techniques many different methodshave been evolved all of which include the essential step of bringingfinely-divided toner particles into proximity with the latentelectrostatic image on the xerographic plate. One of the most successfulmethods and one which is most widely used commercially today is termedcascade development and is described in U.S. Patents 2,618,551 toWalkup, 2,618,552 to Wise, and 2,638,416 to Walkup and Wise. Othertechniques include brush development as described in U.S. Patents2,975,758 to Bird and 2,880,- 699 to Simmonds; liquid spray developmentas described in U.S. Patent 2,551,582 to Carlson; loop development asdescribed in U.S. Patent 2,761,416 to Carlson; and, donor development asdescribed in U.S. Patent 2,895,847 to Mayo.

One xerographic development technique first described by Carlson in U.S.Patent 2,217,776, and known as powder cloud development, has been foundto be an excellent developing technique especially for continuous tonereproduction and other work requiring high resolution. In the powdercloud development technique the finely-divided marking particles ortoner is uniformly suspended or dispersed in a gaseous carrier in theform of an aerosol. These particles are given a substantially uniformelectrostatic charge either before, during, or after the formation ofthe aerosol and then this aerosol or powder cloud is presented to thesurface of an electrostatic image-bearing member such as xerographicplate thus allowing the electric fields set up by the latentelectrostatic image to attract toner particles out of the aerosol andonto its own surface to thereby render the image visible.

Although much effort has been invested in the improvement of powdercloud development as is evidenced by U.S. Patents 2,725,304 toLandrigan, 2,862,646 to Hayford, 2,918,900 to Carlson, and 2,943,950 toRicker, which is only an exemplary sample of the patents relating tothis development technique, the technique is not widely used incommercial machines, one of the reasons apparently being its relativecomplexity and cost. Generally these devices include a source of powderor toner, and means to suspend it in a gas in the form of an aerosolwhich is accomplished by agitating the powder in a gaseous atmosphereand allowing the gas to expand. Depending upon the particular powdercloud generator utilized, the creation of the aerosol may require theutilization of vanes or beaters to stir up the powder, sources of highpressure gas such as compressors, regulating valves, and the like, andmeans to handle and convey the aerosol after its creation while it isbeing transmitted for presentation to the latent electrostatic image tobe developed.

It is an objective of this invention to describe a novel powder clouddeveloping apparatus utilizing a minimum number of parts whicheliminates or substantially reduces the necessity for moving parts.

A further object of this invention is to describe a novel powder clouddeveloping apparatus which forms the developing cloud in situ thuseliminating the problem of conveying the powder cloud from the generatorto the developing station.

It is also an object of this invention to describe a novel powder clouddeveloping method.

The above and still further objects, features, and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed disclosure with specific embodiments of theinvention, especially when taken in conjunction with the accompanyingdrawings wherein:

FIGURE 1 is a cross-sectional view of apparatus for developing a latentelectrostatic image according to the method of this invention.

.! FIGURE l-A is a top isometric view of the aerosol generator of FIGURE2.

FIGURE 2 is a cross-sectional view of a continuous automatic xerographiccopying machine utilizing the developing technique of this invention.

FIGURE 3 is an isometric view of the developercarrying belt of FIGURE 2.

FIGURE 4 is a cross-sectional view of modified apparatus for developinga latent electrostatic image according to the method of this invention.

FIGURE 5 is an isometric view of a cylindrical form of the inventionand,

FIGURE 6 is an isometric view of a polygonal form of the invention.

Referring now to FIGURE 1 of the drawings there is illustrated axerographic plate 11 hearing a latent electrostatic image in positionfor development according to the technique of this invention. It shouldbe noted that the invention is not restricted to the development ofxerographic plates and that any latent electrostatic imagebearing membermight therefore be substituted for xerographic plate 11 and be developedaccording to this invention. Thus, for example, a dielectric sheetbearing a latent electrostatic image placed upon it by tesiprinting asdescribed in U.S. Patent 2,919,967 to Schwertz, might be substituted forxerog-raphic plate 11. As is well known to those skilled in the art ofxerography, the xerographic plate 11 is made up of a photoconductiveinsulating layer 12 such as amorphous selenium, anthr-acene, cadmiumsulfide, zinc oxide in an insulating film-forming hinder, or

the like, on a relatively conductive backing 13 such as aluminum, brass,a conductive plastic conductively coated glass, a paper layer and thelike which is of adequate strength to act as a support for the plate asa whole. In this instance plus marks are shown on the photoconductiveinsulating layer 12 to represent the invisible latent electrostaticimage on the plate. 11 is supported with its photoconductive insulatingsurface facing downwardly on two insulting support blocks 14 and 15 sothat the plate is spaced on the order of about As above the surface ofimpermeable development member 17. The member is made up of a conductivebase plate 18 supporting a dielectric interlayer, 19 which in turnunderlies a conductive grid 21. Conductive base 18 may generally befabricated of any good conductive material such as aluminum, cop-per, orthe like, while dielectric interlayer 19 may be fabricated of any gooddielectric material and its thickness may be varied according to itsdielectric strength, the voltage applied across it, and the spacingdesired between conductive base layer 18 and grid 21. Generally, a thin:interlayer 19 having a high dielectric strength would be most desirablefor this layer so as to maximize the electric field extending up throughthe dielectric between adjacent portions of grid 21 when a voltage isapplied across the conductive layer and grid. In actual practice a sheetof two mil thick Teflon (a trademark of E. I. du Pont & Co. forpolytetrafluorethylene) was found to be quite satisfactory. Grid 21 asshown in FIGURES l and 1-A is made up of a number of parallel conductivelines which are interconnected at their ends by a transverse conductiveline 22. Since this grid is maintained at a grounded reference potentialby wire 16 during opera-tion of the system, it may be connected to theconductive backing 13 of plate 11 which may also be grounded and isshown in FIGURE 1. It has been found that the spacing of the parallelconductive lines which make up grid 21 is not critical to the ope-rationof the process. Thus, for example, grids having 25 lines per inch andgrids having 50 lines per inch have been found to produce good resultsand grids of other densities or shapes may also be used. Conductive baselayer 18 is connected to a double-throw switch 23 so that this baselayer may be connected to either of two potential sources 24 and 26which are of opposite polarity. In operation,

the xerographic plate 11 is unformly charged and exposed to an image ofactivating electromagnetic radiation to form a charge pattern of thetype illustrated in FIGURE 1. The polarity of the initial chargeutilized on the photoconductor is dependent upon the nature of theparticular photoconductor selected for use in the plate 11. Thus, forexample, it has been found that it is preferable to use a positivecharge with amorphous selenium photoconductive insulators while it ispreferable to use a negative charge with zinc oxide binder typephotoconductive insulators. In the event that it is desired to'developthe charged areas of plate 11 development member 17 is loaded withdeveloping material having a polarity opposite to that of the chargedareas. In this instance, then, developing will be carried out withnegatively charged developing particles which will be referred tohereinafter as negative toner. These toner particles are generally lessthan about microns in diameter on an average and are fabricated ofinsulating thermoplastic resins of the types described in U.S. Patents2,788,288, 2,891,011, 2,659,670, and Reissue Patent 25,136- Carlson andalso include some type of pigment such as carbon black. It should .benoted, however, that any particle capable of acquiring and holding acharge may be utilized in this invention.

It is axiomatic in electrostatics that charges of like polarity repeleach other while charges of opposite polarity attract each other. Thus,it is difiicult to deposit a great number of toner particles 27, allcharged to the same polarity between the parallel lines of conductivegrid 21 as shown in FIGURE 1 since the particles tend to repel Thexerographic plate a 4 each other. In order to overcome this diflicultythe bottom conductive plate 18 of member 17 is held at a potential aboveground which is opposite in polarity to the charge on the particlesbeing deposited on the upper surface of dielectric layer 19 between theparallel conductive lines of grid 21 during loading of particles on thegenerator. Since the particles being utilized to develop the positiveimage on xerographic plate 11 are negative a positive potential isapplied to conductive layer 18 thus servingv to attract these particlesand overcoming their tendency to repel each other away from the surfaceof the dielectric layer 19, and allowing a relatively heavy layer ofparticles to be deposited. Any one of a number of well known techniquesmay be utilized for uniformly charging the toner particles to onepolarity and depositing them on the member 17 while conductive backinglayer 18 is connected to its potential source. For example, the cascadetechnique as more fully described in U.S. Patents 2,618,551 to Walkup,U.S. 2,618,552 to Wise, and 2,638,416 to Walkup and Wise which isordinarily utilized to develop a latent electrostatic image may be usedin this instance to load aerosol development member 17. In this loadingprocess grossly larger granular carrier beads are mixed in with thefinely-divided electroscopic toner particles. The rubbing together ofthe granular carrier material with the toner particles in thisdeveloping mixture serves to charge the carrier-particles and the tonerparticles to opposite polarities by triboelectrification causing thetoner particles to attach themselves to the outer periphery of thecarrier beads. The carrier beads are fabricated of a material which isselected from a position in the triboelectr-ic series which is eitherabove or below the toner material depending upon the polarity of chargedesired on the toner particles. When the developing mixture is droppedon or cascaded over member 17 the developing particles are pulled offthe surface of the carrier beads by virtue of the stronger attractiveelectric fields emanating from conductive plate 18 which, during theloading step, is connected to a potential source of opposite polarity tothe charged toner particles.

In addition to the cascade loading technique described above, the tonermay be deposited upon member 17 using a fur brush. In the fur brushtechnique the bristles of the brush are analogous to the carrier beadsof the cascade technique, since .the toner particles are alsotriboelectrically charged to a polarity determined by the relativeposition of the toner particles and the fur fibers in the triboelectricseries. The brush is run through a supply of toner particles which forma coating on the bristles of the fur clinging thereto by reason of theelectrostatic attraction between the toner and the fur just as the tonerclings to the surface of the granular carrier particles. The brush isthen rubbed against the generator which pulls toner particles off of thebristles with the stronger electric field emanating from conductiveplate 18. Other loading techniques may also be utilized.

Once member 17 has been loaded with toner particles 27 as shownin FIGURE1, it is brought adjacent to the latent electrostatic image to bedeveloped so that the spacing between the generator and the plate is onthe order of about /8". The positive potential is then removed frombacking plate 18 by disconnecting it from potential source 26 withswitch 23 thus creating an unstable condition on the surface of thegenerator because of the great number of closely adjacent tonerparticles all having the same charge polarity. Owing to the mutualrepulsion of these particles, many of the particles are rapidly forcedaway or blown off from the surface of the generator thus forming anaerosol in the space between the generator and the xerographic plate 11being developed. Charged particles in this aerosol are picked up by theelectric fields set up by the charge pattern on the photoconductiveinsulating portion of plate 11 thus serving to develop or make visiblethe charge pattern with these toner particles. I

The plate is then removed from the developing system and the now visibleimage is utilized as desired. For. example, the toner image may betransferred to a sheet of copy paper to which it is then permanentlyaffixed by the action of heat or a solvent vapor, or if thephotoconductor is of the non-reuseable type,- the powder image may befixed directly on the surface of the photoconductive insulating member.Alternatively, the image may merely be viewed in situ on thephotoconductive insulator and then removed by brushing, blowing, or thelike, to condition it for reuse.

It has also been found that repulsion of the toner particles from thesurface of member 17 may be improved by connecting conductive base 18 ofthe member to a potential source opposite in polarity to that utilizedduring the loading step rather than merely grounding this base. In thismanner the repulsive force of a field emanating from conductive plate 18is added to the force of mutual repulsion between the toner particlesthereby propelling them into the aerosol with greater velocity anduniformity. This may be accomplished by merely moving switch 23 frompotential source 24 to potential source 26 at the time of developmentrather than merely connecting it to ground. It has been found thatwidely varying voltages may be utilized with this technique of powdercloud generation both during the loading step and during the creation ofthe powder cloud hereinafter referred to as the blowoff step. Thus, forexample, from 200 to 1,000 volts have been found adequate for loadingand these voltages could be much higher if for some reason a muchthicker layer of toner were needed on the generator. As stated above, ashorting or grounding of the plate base has been found adequate for theblowoff step but voltages running up from ground to 1,000 volts and morehave been found to improve blowofi. Obviously, much higher voltagescould be used during both the loading and blowofi steps; however, goodimages have been formed Within these voltage ranges and practicalconsiderations of cost and safety militate against the use of muchhigher voltages than these.

Since toner particles are held on the generator after loading by theforce of attraction of the electrical field emanating from the generatorthe orientation of the loaded generator with respect to the plate to bedeveloped is not critical and it may be above, beneath, or beside theplate. It is to be noted however that the preferred positions of theloaded generator are beneath or beside the plate so that no developingparticles fall on the plate being developed during or after the blowotfmerely by virtue of gravitational forces. These preferred positions thusreduce or eliminate the deposition of toner particles in background ornon-image areas.

As should be apparent from the explanation of the invention given abovethe basic objective of the developing techniques of the instantinvention is to gather a plurality of similarly charged developingparticles closely together in groups which cover the surface of anaerosol generator by applying a holding or attracting force to theparticles and then releasing the attractive force on the particles whenthe generator is opposite the plate to be developed so that mutualrepulsion of the particles forms a powder developing cloud'or aerosol.If in addition to being electroscopic in nature the particles alsocontain a magnetic component the attracting or holding force applied toload or keep the similarly charged particles on the generator prior todevelopment may take the form of a magnetic field from magnets 25 asshown in FIGURE 4. It should be noted however that electric fieldattraction is generally 'preferable since the field may be reversed toaid the forces of mutual particle repulsion during the aerosol formationas described above. In this preferred embodiment the overlying grid 21is generally used to break up the electric field lines of forceemanating from the biased gener-v ator backing 18 during loading andblowofl. so as to create the characteristically strong fringing fieldswhich exist at any point of high potential contrast in a field source.The grid also serves to concentrate the developing particles moreeffectively because they are attracted to areas between the grid linesby the fields from the backing 18 rather than to the ground grid.Clearly a simpler, cheaper and less effective generator operating on theelectric field attraction principle may be fabricated by eliminating thegrid.

The method of this invention may also be applied to an automaticcontinuous xerographic copying machine as illustrated in FIGURE 2. Inthis embodiment a cylindrical xerographic plate 28 which mayalternatively be a flexible endless belt, made up of a photoconductiveinsulating layer '29 on a cylindrical backing 31 is utilized and thisdrum is rotated in the direction indicated by the arrow in theillustration. As the drum rotates it passes a charging corotron 32 whichis connected to a source of high potential so as to uniformly charge thephotoconductive insulating layer 29 of drum 28 by corona discharge asmore fully described in US. Patents 2,836,725 to Vyverberg, and2,777,957 to Walkup. The drum then passes a projector 33 which exposesthe charged photoconductor to a light image of the original to bereproduced thus discharging portions of the charged photoconductor whichare struck by light. The drum then continues to rotate, moving past thedeveloping unit 34 which utilizes the developing technique of thisinvention. The developing unit consists of an enclosure 36 whichsurrounds the entire unit except for that portion adjacent the drum.Within enclosure 36 is a trough 37 containing a quantity of tonerparticles or developer 38. This trough 37 is placed directly beneath arotating brush 3-9 which picks up toner from the trough, charges it, anddeposits it on an endless belt 41 which is entrained about two rollers42 and 43, at least one of which is driven in the direction indicated bythe arrows. As illustrated, most clearly in FIGURE 3, impermeableendless belt 41 is made up of a plurality of powder cloud generators or"the type described in connection with FIGURES 1 and l-A, with eachgenerating unit being electrically separated from all others. Thus, theconductive plate on the back or inside of the generator which isanalogous to conductive plate 18 in FIGURE 1 is broken up into a numberof conductive plates 44, 45, 46, etc., and only those conductiveparallel grid lines on the opposite side of the endless belt dielectriclayer which are above one of the plates 44, 45, or 46 are connectedtogether. The interconnection of the parallel conductors 50 making upthe grids on the outside of endless belt 41 is shown at 47 and 48. Eachof these transverse conductors 47 and 48 connect together a group ofparallel conductors 50 corresponding to one portion of the conductivebacking such as 44, 45, or 46.

In operation rollers 42 and 43 move in the direction indicated by thearrows in FIGURE 2, at least one of them being driven so as to moveendless belt 41 around their peripheries. During rotation, portions ofthe belt are also in sliding contact with contacts 49 and 51. If brush39 is being used to deposit negatively charged toner on the belt todevelop a positive image on photoconductive insulator 29, conductiveroller 42 is connected to a source of potential of positive polaritywhile contact 49 is connected to ground. Since roller 42 is connected toa positive source of potential it attracts this negative toner throughits connection with the backing plates 44, 45, 46, etc., with which itmay be in contact at that particular time, while grounding of contact 49serves to ground all grid sections above these conductive backingportions by contacting transverse conductors similar to 47 and 48' asshown in FIGURE 3. This gives the endless generator belt a uniform anddense coating of toner particles which are held to its outer surface byelectric field attraction. Upon continued rotation of rollers 42 and 43endless belt 41 passes 7, over sliding contact 51 which is connected toa potential source opposite in polarity to that applied to roller 42,the potential source in this instance being negative. The potentialsource applied to conductive backing portions similar to 44, 45, and 46,is thus changed from positive to negative thereby eliminating the tonerattracting positive electric field produced by the positive potentialsource and substituting in its place a repelling negative electric fieldwhich is produced by conductive portions 44-46, which are in contactwith element 1.

The combined effect of mutual repulsion between the toner particles ofthe same polarity and the repelling fields set up by the negativepotential applied to the conductive backing portions serves to repel thetoner particles into the space between electrostatic image-bearing drum28 and belt 4 1, as described more fully in connection with FIGURE 1.This results in the production of a powder aerosol which serves todevelop the latent electrostatic image on electrostatic image-bearingmember 28, giving superior reproduction of continuous tone and soliddarkoriginal subjects. Drum 28 then continues its rotation with a developedparticle image on its surface coming into contact with a web of copypaper 52 from a supply roll 53 which is held against the drum by twoidle rollers 54 and 56. Behind and spaced slightly from this copy sheet52 between idle rollers 54 and 56 is a transfer unit 57. This transferunit is a corona generating filament or filament array similar to thecharging corotron described above connected to a source of highpotential opposite in polarity to the toner particles utilized todevelop the image. As described more fully in US. Patent 2,576,047 toSchalfert, this corona generating unit deposits positive charge on theback of the copy -web 52 thus serving to attract the toner particles andremove them from the surface of drum 28 transferring them to the copysheet 52. Copy sheet 5 2 then passes under a fixing unit 58 which servesto permanently aflix the powder image to the copy web. In this instancea heat fixer of the type which fuses the thermoplastic developingparticles to the copy Web is illustrated, however, any of the well knownfixing techniques such as solvent vapor spray, adhesive overcoalting, orthe like, may be used to fix this image on the copy sheet. The copysheet is then rolled up on a take-up roll 59'for future use.

It should be recognized that many alternative materials and techniquesmay be utilized in carrying out the instant invention. For example,toner particles charged to the same polarity as the latent electrostaticimage to be developed may be utilized so as to give a negativereproduction of the original. In this case the polarity of the voltageapplied to the conductive backing of the generator is still the oppositein polarity to the charge on the toner particles for generator loadingand of the same polarity as the charge on the toner particles for theblowofi step. Variations may also be made in the generator fabricationand configuration including changes in the grid such as varying thenumber of grid lines per inch, changing the shape of the grid to ascreen mesh, a spiral mesh, or any one of many other shapes. Thegenerator may also be fabricated in forms other than the plate of FIGURE1 or the endless belt of FIGURE 2. For example, it might be madedirectly on a cylindrical drum 60, as in FIGURE 5 which is continuouslyrotated during the process or on the surface of a polygonal support 70,as in FIGURE 6, which is indexed around a central shaft for loading andblowotf.

While the specific embodiments shown and described in this specificationand drawings are admirably adapted to fulfill the stated objects, itshould be understood that it is not intended to confine the invention tothese disclosed embodiments since the invention itself is susceptible ofembodiment in many various forms all coming within the scope of thefollowing claims.

What is claimed is:

1. A method of developing a latent electrostatic image comprisinggathering a plurality of fine-divided electroscopic marking particles,substantially all of which are charged to the same polarity closelytogether on the surface of an aerosol generator by applying anattracting force to said particles, bringing the particle covered sideof said aerosol generator into a closely spaced relationship vvith thelatent electrostatic image to be developed and then releasing theapplied attracting force on said particles whereby they are mutuallyrepelled by the similarity of their charge into the space between saidgenerator and said image to form an aerosol and develop said image.

2. A method according to claim 1 including the further step of applyinga force repelling said particles from said generator in addition totheir mutual repelling forces after releasing said attracting force.

3. A method according to claim II in which said attracting force is amagnetic field and said marking particles are magnetically attractable.

4. A xer-ographic powder aerosol development generator comprising incombination,

a unitary powder impermeable development member adapted to attract, holdand subsequently to release like charged toner particles comprising anelectrically continuous conductive grid at a first surface thereof,

an electrically conductive backing member exclusively at the secondsurface thereof,

a thin solid insulating layer separating said grid and backing memberand in contact therewith,

and reversible polarity electrical biasing means connected between saidgrid and said backing member to initially attract and hold like chargedtoner particles by a first polarity and subsequently to release saidparticles by a second polarity.

5. A xerographic powder aerosol development generator comprising incombination,

a unitary powder impermeable development member comprising anelectrically continuous conductive grid separated from an electricallyconductive backing member solely by a thin solid insulating layer,

means to deposit finely divided electroscopic marking particlessubstantially all of which are charged to the same polarity over saidgrid and onto said insulator where uncovered by said grid,

electrical bias means operably connected to said grid and backing memberto electrically bias said backing member wit-h respect to said gridwhile said particles are being deposited thereon whereby said markingparticles are held against the surface of said insulating layer whereuncovered by said grid,

and electrical bias means operably connected to said grid and backingmember to thereafter electrically bias said backing member with respectto said grid whereby said particles are no longer held against thesurface of said insulating layer and are mutually repelled by thesimilarity of their charge and by the like-charge on the biasedconductive backing layer thereby forming a powder aerosol adjacent thedevelopment member.

6. A xerographic powder aerosol development generator comprising incombination,

a unitary powder impermeable development member comprising anelectrically continuous conductive grid separated from an electricallyconductive backing member solely by a thin solid insulating layer,

means to deposit finely divided electroscopic marking particlessubstantially all of which are charged to the same polarity over saidgrid and onto said insulator where uncovered by said grid,

electrical bias means operably connected to said grid and backing memberto electrically bias said backing member with respect to said grid whilesaid particles are being deposited whereby said marking the similarityof their charge and by the like charge on the bias conductive backinglayer into the space between said development member and said latentimage bearing member and deposit on said latent image bearing member inaccordance with the electrostatic latent image thereon.

7. A xerographic powder aerosol development generator comprising incombination,

a unitary powder impermeable development member comprising anelectrically continuous conductive grid separated from an electricallyconductive backing member solely by a thin solid insulating layer,

means to deposit finely divided electroscopic marking particlessubstantially all of which are charged to the same polarity over saidgrid and onto said insulator where uncovered by said grid,

electrical bias means operably connected to said grid and backing memberto electrically bias said backing member to a first polarity withrespect to said grid while said particles are being deposited wherebysaid marking particles are held against the surface of said insulatinglayer where uncovered by said grid,

means to bring said grid into a closely spaced apart upwardly facingrelationship with an electrostatic latent image bearing member havingthereon a latent electrostatic charge pattern distinct from said grid,

means to maintain said grid at a fixed potential with respect to saidimage bearing member,

and bias means operably connected to said grid and backing member tothereafter electrically bias said backing member to a second polaritywith respect to said grid whereby said particles are no longer heldagainst the surface of said insulating layer and are mutually repelledby'the similarity of their charge and by the like charge on the biasconductive backing layer into the space between said development memberand said latent image bearing member and deposit on said latent imagebearing member in accordance with t-he electrostatic latent imagethereon.

8. A xerographic powder aerosol development generator comprising incombination,

a unitary powder impermeable development member comprising an endlesselectrically insulating web hearing on its outer surface and in contacttherewith a plurality of mutually insulated electrically continuousconductive grid patterns and bearing on its inner surface and in contacttherewith a plurality of mutually insulated electrically conductiveareas, each of said grid patterns being substantially coextensive with aconductive area,

deposition means to deposit finely divided electroscopic markingparticles substantially all of' which are charged to the same polarityover the outer surface of said web,

first electrical contact means to contact said grid patterns and saidcontinuous areas where subject to said deposition means,

electrical bias means connected to said first contact means toelectrically bias said continuous areas to a first polarity with respectto said grid patterns while said particles are being deposited wherebysaid marking particles are held against the outer surface of said webwhere uncovered by said grid patterns,

moving means to bring the outer surface of said web into a closelyspaced-apart upwardly facing relationship with an electrostatic latentimage bearing member having thereon an electrostatic charge patterndistinct from said grid, 1

second contact means to contact said grid patterns and said continuousareas where adjacent to said image bearing member,

means connected to said second contact means and to said image bearingmember to maintain said grid patterns at a fixed potential with respectto said image bearing member when adjacent to said image bearing member,

and bias means connected to said second contact means to electricallybias said continuous areas to a second polarity with respect to saidgrid patterns whereby said particles are no longer held against theouter surface of said web and are mutually repelled by the similarity oftheir charge into the space between said web and said latent imagebearing member and deposit on said latent image bearing member inaccordance with the electrostatic latent image thereon.

References Cited by the Examiner UNITED STATES PATENTS 2,109,333 2/ 1938Formhals.

2,752,833 7/1956 lac-ob 118-637 X 2,787,556 4/1957 Haas 11717.52,839,400 6/ 1958 Moncrief-Yeates 118-637 2,895,847 7/1959 Mayo 118-637X 2,910,351 10/1959 Szpak et al. 118-637 X 2,940,864 6/1960 Watson117-17 2,959,153 11/1960 Hider 118637 3,011,473 12/1961 Gundlach -1118637 3,093,039 6/1963 Rheinfrank 11717.5 X

WILLIAM D. MARTIN, Primary Examiner.

G. L. HUBBARD, Assistant Examiner.

1. A METHOD OF DEVELOPING A LATENT ELECTROSTATIC IMAGE COMPRISINGGATHERING A PLURALITY OF FINE-DIVIDED ELECTROSCOPIC MARKING PARTICLES,SUBSTANTIALLYALL OF WHICH ARE CHARGED TO THE SAME POLARITY CLOSEYTOGETHER ON THE SURFACE OF AN AEROSOL GENERATOR BY APPLYING ANATTRACTING FORCE TO SAD PARTICLES, BRINGING THE PARTICLE COVERED SIDE OFSAID AEROSOL GENERATOR INTO A CLOSELY SPACED RELATIONSHIP WITH THELATENT ELECTROSTATIC IMAGE TO BE DEVELOPED AND THEN RELEASING THEAPPLIED ATTRACTING FORCE ON SAID PARTICLES WHEREBY THEY ARE MUTUALLYREPELLED BY THE SIMILARITY OF THEIR CHARGE INTO THE SPACE BETWEEN SAIDGENERATOR AND SAID IMAGE TO FORM AN AEROSOL AND DEVELOP SAID IMAGE.